The present invention relates generally to x-ray tubes, and, more specifically, to cooling thereof.
An x-ray tube includes an evacuated glass enclosure in which is mounted an anode target adjacent to a cathode. The target is a circular disk formed of a suitable metal or graphite or both, and is mounted to a drive shaft of a motor for rotating the target at high rotational speeds, such as about 10,000 rpm. Formed on the front face of the target is an annular focal track against which electrons from the cathode are bombarded for creating the x-rays which are emitted through the sidewall of the enclosure. The impinging electrons heat the focal track and in turn the target to substantially high temperature during operation. The x-ray tube therefore requires cooling which is typically accomplished by circulating a cooling fluid such as oil around the glass enclosure for removing the heat therefrom.
However, since a high vacuum is maintained inside the glass enclosure, heat transfer from the target to the oil surrounding the enclosure is effected primarily by thermal radiation. A typical metallic target is made of a conventional TZM material which is a molybdenum alloy with zirconium and titanium, and often includes an emissivity enhancing coating to improve thermal radiation at the high operating temperature. Targets may also be formed of graphite which inherently have relatively high emissivity without an additional emissivity enhancing coating. And, targets may be formed of both TZM and graphite suitably brazed together.
The targets are typically machined to the required final dimensions, with the machining of the graphite targets providing an outer surface from which graphite particles may be released during operation. This is undesirable since released graphite particles in the evacuated glass enclosure would degrade performance of the x-ray tube. Accordingly, graphite targets require a pyrolytic carbon infiltration (PCI) coating to prevent the liberation of graphite dust. This coating, however, can significantly reduce the emissivity of the graphite from a nominal value of about 0.825 down to as low as 0.4 depending on deposition conditions.
Due to the limited ability to effectively cool the x-ray tube target, the x-ray tube must therefore be operated intermittently in a corresponding duty cycle which ensures that the target does not exceed a predetermined operating temperature that would lead to decreased useful life of the x-ray tube. It is therefore desirable to provide enhanced cooling of the target for improving the operating duty cycle of the x-ray tube.